The Doctor's bio-data extract is stolen from the Matrix on Gallifrey.
Soon after, a being from an anti-matter universe begins to genetically
bond with the Doctor. He and Nyssa return to Gallifrey, only for the
High Council to order his execution -- while on Earth, unbeknownst to
her friends, Tegan's search for her missing cousin in Amsterdam is
somehow tied into the events as well. It is left to Nyssa to uncover the
identity of a traitor on the High Council, and to unveil the enemy
manipulating the Doctor -- an entity who has long thirsted for revenge
against both the Doctor and the Time Lords themselves.

Production

Script editor Eric Saward had greatly enjoyed Johnny Byrne's first
Doctor Who serial, The Keeper Of
Traken, which aired as part of Season Eighteen. Unaware that
that story had been heavily rewritten by former script editor
Christopher Bidmead, Saward invited Byrne to submit ideas for Season
Twenty on September 30th, 1981. It was thought that Byrne might
contribute the year's opening serial, which meant that his storyline
would have to deal with a number of requirements.

First, producer John Nathan-Turner wanted to take Doctor Who
overseas for just the second time in its history. In 1979, City Of Death had been filmed on location in
Paris. Nathan-Turner felt that Amsterdam would be similarly affordable,
especially since it was already one of the locales used by the BBC soap
opera Triangle. In order to justify the trip, Byrne was asked to
make the Dutch city a key aspect of his storyline.

Tegan was left behind on Earth in the cliffhanger ending
to Time-Flight, but no specific resolution
had been planned

Second, Byrne had to resolve the ending of the Season Nineteen finale,
Time-Flight, in which Tegan had been
inadvertently left behind on Earth. This cliffhanger had been designed
to maintain viewer interest during the nine-month break between seasons,
but had been planned without any specific resolution in mind. As a
result, it would be Byrne's responsibility to find a way to reunite
Tegan with the Doctor and Nyssa. It was suggested that the suspense
could be prolonged by deferring Tegan's reintroduction until the second
episode of Byrne's adventure.

When he was contacted by Saward, Byrne was already toying with an idea
in which an alien uses a bridge between dimensions, called the Arc of
Infinity, to cause time shifts in London. Byrne felt that this concept
could be relocated to Amsterdam, and it was suggested that Tegan might
be holiday there, where she stumbles upon some form of criminal
activity which would require the Doctor's attention. (The production
team stipulated that this should not involve drug smuggling, the theft
of diamonds or Old Dutch Masters, or anything political).

On December 15th, Byrne submitted a storyline entitled “The Time
Of Neman”. This involved the Doctor suffering nightmares about his
regeneration, which were actually a precursor to the arrival in our
universe of an entity called the Avatar, who takes on the Doctor's form
and goes to Amsterdam. Operating there as Neman (a name recycled from
The Keeper Of Traken), the Avatar begins to
take control of human minds, striving to create a form in which it will
be able to permanently maintain its existence. The Doctor and Nyssa
discover Neman's plot when they land in a future version of Amsterdam,
which is populated by robot guards called Sweepers, the elderly
Resisters, and barbaric Anarchs. Realising that history has been
altered, they travel back in time to present-day Amsterdam. There, the
Doctor discovers that Time Lord regeneration is the mechanism by which
the Avatar is made manifest. The Avatar is defeated when the Doctor
relives his own recent regeneration.

The production team was concerned that the Amsterdam
location was essentially incidental to the plot

Nathan-Turner and Saward had immediate concerns about “The Time
Of Neman”, most notably the fact that the Amsterdam location was
essentially incidental to the plot. It was also felt that the importance
of the Doctor's dreams harkened too closely to elements of Christopher
Bailey's Snakedance, which was intended to
follow “The Time Of Neman” in the transmission order.
Furthermore, aware of the success of the Master's return in The Keeper Of Traken, Nathan-Turner wanted his
season premiere to bring back another old enemy, and nominated several
returning villains as possible replacements for the Avatar. One of these
was Omega, a renegade Time Lord who had last appeared in the
tenth-anniversary story The Three Doctors,
whose inclusion was proposed by the Doctor Who production
office's unofficial fan adviser, Ian Levine. Although he was not keen on
any of the suggestions, Byrne reluctantly agreed to amend his storyline
to include Omega. This pleased Saward, who wanted to revisit Gallifrey
during Season Twenty.

Now called “The Time Of Omega”, Byrne's scripts were
commissioned on January 13th, 1982. As work on them proceeded, it was
decided that Sarah Sutton would leave Doctor Who partway through
Season Twenty. To pave the way for Nyssa's departure, Byrne was asked to
write the character as being more mature than previously depicted.
Another reference to past occurred when the Lord President, originally
unnamed, was identified as Borusa, last seen as a Cardinal in The Invasion Of Time. Meanwhile, Cardinal
Zorac was originally called Zoral.

Increasingly aware of Doctor Who fandom, Nathan-Turner decided
that he wanted Omega's return to be a surprise for them, and so, around
the end of February, the title of Byrne's adventure became Arc Of
Infinity. The scripts began to arrive around this time, and
unfortunately the production team was still dissatisfied with Byrne's
handling of the Amsterdam setting. Although the writer had tried to
justify the location by including elements such as a fusion booster
which only functions below sea level, it was felt that Byrne had not
made Amsterdam sufficiently integral to the story. No obvious solution
to this problem presented itself, however, and so no significant changes
were made to Byrne's scripts.

Although Arc Of Infinity was planned as the first story of Season
Twenty, it was agreed that it should follow Snakedance into production, so that the
Amsterdam shoot would take place in the warmer May weather. As a result,
Arc Of Infinity was designated Serial 6E. Its director would be
Ron Jones, who had last worked on Time-Flight. As pre-production progressed,
Nathan-Turner encouraged costume designer Dee Robson to deviate from the
way Omega had appeared in The Three
Doctors. Nathan-Turner thought that this would further help
preserve the surprise of the villain's identity, and could be justified
by Omega's nigh-total control over mass and form in his antimatter
domain.

Colin Baker was disappointed that accepting the role of
Maxil meant that he could never play the Doctor himself

Meanwhile, amongst the guest artistes cast by Jones in Arc Of
Infinity was Colin Baker, playing Maxil. Baker had been suggested to
Jones by assistant floor manager Lynn Richards, on the basis of his role
in the Blake's 7 episode City At The Edge Of The World.
Although eager to appear in Doctor Who, Baker was disappointed at
the thought that he could now never play the Doctor himself. Ironically,
just over a year later, Baker would find himself offered the role of the
Sixth Doctor...

Location filming in Amsterdam took place between May 3rd and 7th. A
number of venues were employed, especally for the chase sequence in
episode four, which Jones had largely composed himself while
reconnoitering possible locations. Crowd control proved to be a major
issue throughout the shoot, with Dutch viewers recognising Peter Davison
from his role as Tristan Farnon in All Creatures Great And Small.
Nathan-Turner tried to assist Jones in dealing with the onlookers, and
indeed is visible in the transmitted version of Arc Of Infinity,
shooing away a group of gawkers in the background of the episode four
scene in which the Doctor and Nyssa use a telephone box. At one point,
Nathan-Turner's attempts at crowd control became dangerous when an
elderly woman assumed he was a thief and attacked him. Meanwhile, on the
night of the 6th, several members of the cast and crew decided to take a
walk around Amsterdam. Unfamiliar with the city, they suddenly found
themselves in its infamous red light district, where Janet Fielding was
mistaken for a prostitute.

Back in London, studio work for Arc Of Infinity began with a
two-day session in BBC Television Centre Studio 1, on May 17th and 18th.
In addition to material in the youth hostel and the cafe, the first day
concentrated on scenes in the TARDIS corridors and console room, during
which Baker found that his plumed headpiece -- which he'd nicknamed
“Esmerelda” -- was too tall to accommodate his entrance
through the TARDIS doors. Consequently, Baker decided to carry the
headpiece for the majority of his screentime. May 18th dealt with
sequences in Nyssa's room, as well as the Amsterdam crypt and its
adjoining passages. The Ergon costume, contracted out to props makers
Imagineering, debuted on this day. The Ergon was envisioned as a
construct made up of bones, reminiscent of the eponymous creature in the
1979 film Alien. Sadly, however, this effect did not come off,
and although there was no time to modify or replace it, the costume was
judged a failure.

The Ergon was meant to be reminiscent of the eponymous
creature from the 1979 film Alien

The second studio block was three days in length, running from May 31st
to June 2nd; again, the venue was TC1. Various sets were employed on the
first day: Hedin's office, Omega's TARDIS, the Matrix, and the
Gallifreyan computer room. Further scenes in the Doctor's TARDIS were
also completed on this day. Additional material in Omega's TARDIS and
the computer room was captured on June 1st, alongside various
Gallifreyan corridors and the security compound where the TARDIS
materialises. Finally, the 2nd was largely concerned with scenes set on
Gallifrey: the Council chamber, the termination area, and the offices of
the Castellan and another Time Lord. An insert of the explosion in the
Ansterdam pump house in was also taped.

Like Season Nineteen, Season Twenty would see two episodes aired each
week. Instead of broadcasts on Mondays and Tuesdays, however, the new
season was scheduled to be transmitted on Tuesdays and Wednesdays in
most of the country. The exception to this rule was the season premiere
-- Arc Of Infinity part one -- which aired on Monday, January
3rd, 1983. In recent times, great pains had been taken to preserve the
identities of returning villains in the closing credits. To this end,
Ian Collier was billed as “The Renegade” at the end of the
the first two installments, with his credit finally reverting to
“Omega” for part three.